The present invention refers highly bi-axially oriented, heat-shrinkable, thermoplastic, multi-layer film and to the process for the manufacture thereof.
More particularly the present invention refers to a heat-shrinkable, thermoplastic, multi-layer film comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer (EVOH), a first outer layer (B) comprising an ethylene homo- or co-polymer and a second outer layer (C), which may be equal to or different from the first outer layer (B), comprising an ethylene homo- or co-polymer, characterized in that said film has been bi-axially oriented at an orientation ratio in the longitudinal direction higher than 4:1, preferably higher than 4.5:1, even more preferably of at least 5:1, and at an orientation ratio in the cross-wise direction higher than 4:1, preferably higher than 4.5:1, even more preferably of at least 5:1.
Bi-axially oriented, heat-shrinkable, thermoplastic, multi-layer films are films that have been oriented by stretching in two perpendicular directions, typically the longitudinal or machine direction (MD) and the transverse or crosswise direction (TD), at a temperature higher than the highest Tg of the resins making up the film layers and lower than the highest melting point of at least one polymer of the film layers, i.e. at a temperature where the resins, or at least some of the resins, are not in the molten state.
Bi-axially oriented, heat-shrinkable, thermoplastic films are made by extruding polymers from a melt into a thick sheet that is quickly quenched to prevent or delay polymer crystallization, and then oriented by stretching under temperature conditions, as indicated above, where molecular orientation of the film occurs and the film does not tear. Upon subsequent re-heating at a temperature close to the orientation temperature, the oriented, heat-shrinkable, film will tend to shrink in seeking to recover its original dimensional state. In fact, when the film, where the polymer molecules are aligned in the direction of the drawing force and locked into this configuration by cooling, is heated to a temperature close to the orientation one, mobility is restored in the polymer molecules and they relax back to the coil configuration, physically manifesting said relaxation with a shrink along the direction of the orientation.
Orientation brings out the maximum strength and stiffness inherent in the polymer system, thus increasing the tensile properties of the film.
Orientation also induces higher level of crystallinity so that properties like gas barrier properties are further enhanced in an oriented film.
In general orientation leads to a crystalline structure that scatters much less light than the crystalline domains formed in unoriented films and therefore orientation leads to generally superior optical properties.
Oriented, heat-shrinkable films are therefore widely appreciated and widely used in packaging, particularly in food packaging. In general terms the packaging of food and non-food items by means of an oriented, heat-shrinkable, thermoplastic film comprises configuring the heat-shrinkable packaging material, either partially or completely, around a product, removing excess air if necessary, sealing it to itself or to the rims of a support containing the product to be packaged or otherwise let the two edges of the packaging material to overlap and adhere to each other without heat-sealing them and thereafter exposing the package to a heat source thereby causing the heat-shrinkable film to shrink and conform with the contours of the packaged item or become tight between the rims to which it has been sealed.
Heat-shrinkable films are used to both provide the package with an aestethically appealing appearance and guarantee that the packaged product is protected from the environment.
Bi-axially oriented, heat-shrinkable multi-layer films comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer, a first outer layer (B) comprising an ethylene homo- or copolymer and a second outer layer (C) comprising an ethylene homo- or co-polymer, are known.
As an example; EP-A-141,555 discloses an oriented five layered film with a core layer of a blend of an ethylene-vinyl alcohol copolymer and a polyamide, two outer layers of a blend of ethylene-vinyl acetate and low density linear polyethylene, and two tie layers on the two surfaces of the core layer, adhering said surfaces of the core layer to a respective outer layer. EP-A-141,555 describes, as the most practical manner of extruding and orienting the film, the xe2x80x9cdouble-bubblexe2x80x9d technique, according to which the film is extruded downwardly as a tube formed by an annular die, quenched by a water cascade and a water bath, re-heated to the suitably selected orientation temperature, and then oriented by stretching. Stretching in the machine direction is carried out by two sets of rolls that are rotated in such a way so as to establish a linear rate differential therebetween, while the simultaneous orientation in the cross-wise or transversal direction is carried out by inflating the bubble trapped between the to nips of the rolls. Convenient orientation ratios there described are comprised between 2:1 and 4:1 in both directions.
EP-A-217,596 describes an oriented, heat-shrinkable cross-linked film having a core layer comprising an ethylene-vinyl alcohol copolymer, two outer layers comprising a blend of ethylene-vinyl acetate, low density linear polyethylene and medium density linear polyethylene, and two tie layers adhering the surfaces of the core layer to a respective outer layer. The process there described involves extrusion of a thick sheet in the form of a tube, cooling thereof, cross-linking by irradiation, re-heating to the suitably selected orientation temperature and orientation in a way similar to that described in EP-A-141,555. The orientation ratios described in the examples of EP-A-217,596 are about 3.5:1 in each direction.
WO-A-95/13,187 describes an alternative process for the manufacture of bi-axially oriented, heat-shrinkable multi-layer films, including those described in the above patents, having at least one layer comprising an ethylene-a-olefin copolymer and showing more than one melting point in DSC. Said process provides for the extrusion of the polymers through a flat die in the form of a sheet, and after a quenching step and an optional irradiation step, for the heating of the flat sheet to the orientation temperature and the stretching thereof first longitudinally, by running the sheet over at least two series of pull rolls wherein the second set runs at a higher speed than the first one, and then transversally, by grasping the edges of the sheet by clips carried by two continuous chains running on two tracks that move wider apart as they go along.
The films obtained by this latter method distinguish from those obtainable by the trapped bubble technique in the thickness variation, that is always lower than 10% and in the planarity, that is significantly more controlled.
While, in line of principle, higher stretching ratios could be employed using flat extrusion and flat orientation with respect to those obtainable with the trapped-bubble technique, an MD stretching ratio of 2.4 to 1 and a TD stretching ratio of 4.5 to 1 are reported in Example 26 of WO-A-95/13,187 for the manufacture of a film comprising a core layer of a blend of ethylene-vinyl alcohol and 20% by weight of a polyamide 6/12.
Ethylene-vinyl alcohol is in fact a highly crystalline polymer known to be difficult to orient. Particularly in the sequential stretching described in WO-A-95/13,187, the first orientation step induces some polymer crystallization that increases the resistance of the film to further stretching. Ethylene-vinyl alcohol is therefore typically admixed with a polyamide or other plasticizers in the orientation processes.
The use of high stretching temperatures, particularly for the transverse stretching, would help to increase the stretching ratios. As a matter of fact orientation of films with a core layer of ethylene-vinyl alcohol and outer layers of polypropylene or propylene-ethylene co-polymers at high stretching ratios on a sequential tenter frame line using fairly high orientation temperatures is described in the prior art (EP-A-311,293). Such high orientation temperatures however would not be compatible with the presence of an ethylene homo- or co-polymer in the outer layers. Furthermore the use of high orientation temperatures would impair the shrink and mechanical properties of the end film as the higher the orientation temperature the less oriented the end film. As a matter of fact so-called heat-set films (i.e. films heat-stable that would not shrink when heated close to the temperature of orientation) are typically obtained.
Simultaneous stretching of a continuous traveling flat sheet in the longitudinal and transversal directions is a technique known in the literature since many years. U.S. Pat. No. 2,923,966, issued in 1960, described an apparatus for carrying out such a simultaneous flat stretching. The apparatus there claimed comprised two endless conveyors, positioned on the two sides of the web and disposed along divergent paths, said conveyors being formed of a plurality of links pivotally interconnected at their ends to provide a lazy-tongue structure and carrying a series of spaced clips to grip the web edges.
The use of endless loop linear motor systems for the simultaneous stretching of a continuous traveling flat sheet has later been described, e.g. in U.S. Pat. No. 3,890,421, and improvements thereto, with particular reference to the problem of controlling synchronism, have been described in e.g. U.S. Pat. No. 4,825,111, U.S. Pat. No. 4,853,602; and U.S. Pat. No. 5,051,225.
Actually there are various commercial simultaneous bi-axial film tenters.
These are however employed for the manufacture of heat-stable, generally monolayer films, particularly bi-axially oriented polyethylene terephthalate (BO-PET) and bi-axially oriented polyamide (BO-PA).
None of these apparatuses has been employed or tested for the manufacture of heat-shrinkable multi-layer films.
It has now been found that it is possible to obtain a highly bi-axially oriented, heat-shrinkable film comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer, a first outer layer (B) comprising an ethylene homo- or co-polymer and a second outer layer (C), which may be equal to or different from the first outer layer (B), comprising an ethylene homo- or co-polymer, by carrying out the stretching step simultaneously in both directions by means of a simultaneous tenter frame and using orientation ratios higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1, in both the longitudinal direction and the cross-wise direction.
The film thus obtained not only has a thickness variation less than 10% and a very good planarity but also high and fairly balanced shrink properties.
In the packaging of a relatively rigid product which is not distorted by forces produced by a shrinking film, it is generally desirable to provide a heat-shrinkable packaging film with as high a free shrink as possible, in order to provide the xe2x80x9ctightestxe2x80x9d possible packaging over the product and/or to provide the desired shrink at a lower temperature. The highly oriented EVOH-comprising films obtained according to the present invention have a high free shrink, thereby enabling improved product appearance over a film having a lower free shrink.
A first object of the present invention is therefore a bi-axially oriented, heat-shrinkable, thermoplastic, multi-layer film comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer (EVOH), a first outer layer (B) comprising an ethylene homo- or co-polymer and a second outer layer (C), which may be equal to or different from the first outer layer (B), comprising an ethylene homo- or co-polymer, characterized in that said film has been bi-axially oriented at an orientation ratio in the longitudinal direction higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1 and at an orientation ratio in the cross-wise direction higher than 4:1, preferably higher than 4.5:1, a more preferably of at least 5:1.
A second object of the present invention is a process for manufacturing a highly bi-axially oriented, heat-shrinkable, thermoplastic, multi-layer film comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer (EVOH), a first outer layer (B) comprising an ethylene homo- or co-polymer and a second outer layer (C), which may be equal to or different from the first outer layer (ES), comprising an ethylene homo- or co-polymer, which process comprises extrusion of the film resins through a flat die and bi-axial orientation of the obtained cast sheet simultaneously in the two perpendicular directions at an orientation ratio in the longitudinal direction higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1 and at an orientation ratio in the cross-wise direction higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1.
A third object of the present invention is the use of a bi-axially oriented, heat-shrinkable multi-layer film comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer (EVOH), a first outer layer (B) comprising an ethylene homo- or co-polymer and a second outer layer (C), which may be equal to or different from the first outer layer (B), comprising an ethylene homo- or co-polymer, wherein said film has been bi-axially oriented at an orientation ratio in the longitudinal direction higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1 and at an orientation ratio in the cross-wise direction higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1, in the packaging of food or non-food products.
As used herein, the term xe2x80x9cfilmxe2x80x9d is used in a generic sense to include plastic web, regardless of whether it is film or sheet. Typically, films of and used in the present invention have a thickness of 150 xcexcm or less, preferably they have a thickness of 100 xcexcm or less, more preferably a thickness of 75 xcexcm or less, still more preferably a thickness of 50 xcexcm or less, and yet, still more preferably, a thickness of 30 xcexcm or less.
As used herein, the phrases xe2x80x9cinner layerxe2x80x9d and xe2x80x9cinternal layerxe2x80x9d refer to any layer having both of its principal surfaces directly adhered to another layer of the film.
As used herein, the phrase xe2x80x9couter layerxe2x80x9d refers to any layer of film having only one of its principal surfaces directly adhered to another layer of the film.
As used herein, the phrase xe2x80x9cinside layerxe2x80x9d refers to the film outer layer that is closest to the product, relative to the other layers of the multi-layer film.
As used herein, the phrase xe2x80x9coutside layerxe2x80x9d refers to the film outer layer, of a multi-layer film packaging a product, which is furthest from the product relative to the other layers of the multi-layer film.
As used herein, the phrases xe2x80x9cseal layerxe2x80x9d, xe2x80x9csealing layerxe2x80x9d, xe2x80x9cheat seal layerxe2x80x9d, and xe2x80x9csealant layerxe2x80x9d, refer to an outer layer involved in the sealing of the film to itself, to another layer of the same or another film, and/or to another article which is not a film. With respect to packages having only fin-type seals, as opposed to lap-type seals, the phrase xe2x80x9csealant layerxe2x80x9d generally refers to the inside layer of a package.
As used herein, the term xe2x80x9ccorexe2x80x9d, and the phrase xe2x80x9ccore layerxe2x80x9d, refer to any internal layer which preferably has a function other than serving as an adhesive or compatibilizer for adhering two layers to one another.
As used herein, the phrase xe2x80x9ctie layerxe2x80x9d refers to any internal layer having the primary purpose of adhering two layers to one another. Preferred polymers for use in tie layers include, but are not restricted to ethylene-unsaturated acid copolymer, ethylene-unsaturated ester copolymer, anhydride-grafted polyolefin and mixtures thereof.
As used herein, the phrase xe2x80x9cthickness variationxe2x80x9d refers to the percent value obtained by measuring the maximum and minimum thickness of the film, calculating the average thickness value and applying these numbers to the following       Thickness    ⁢          xe2x80x83        ⁢    variation    ⁢          xe2x80x83        ⁢          (      %      )        =                              film          ⁢                      xe2x80x83                    ⁢                      thickness                          (              max              )                                      -                  film          ⁢                      xe2x80x83                    ⁢                      thickness                          (              min              )                                                  film        ⁢                  xe2x80x83                ⁢                  thickness                      (            avg            )                                xc3x97    100.  
The maximum and minimum thicknesses are determined by taking a total of 10 thickness measurements at regular distance intervals along the entirety of the transverse direction of a film sample, recording the highest and lowest thickness values as the maximum and minimum thickness values, respectively, while the average value is determined by summing up the same 10 thickness measurements and dividing the result by ten. The thickness variation is then computed (as a percent value) using the formula above. A thickness variation of 0% represents a film with no measurable differences in thickness. A thickness variation over 20% is unacceptable industrially while a thickness variation below 10% is a good value.
As used herein, the phrase xe2x80x9cmachine directionxe2x80x9d, herein abbreviated xe2x80x9cMDxe2x80x9d, refers to a direction xe2x80x9calong the lengthxe2x80x9d of the film, i.e., in the direction of the film as the film is formed during extrusion and/or coating.
As used herein, the phrase xe2x80x9ctransverse directionxe2x80x9d, herein abbreviated xe2x80x9cTDxe2x80x9d, refers to a direction across the film, perpendicular to the machine or longitudinal direction.
As used herein, the phrases xe2x80x9corientation ratioxe2x80x9d and xe2x80x9cstretching ratioxe2x80x9d refer to the multiplication product of the extent to which the plastic film material is expanded in the two directions perpendicular to one another, i.e. the machine direction and the transverse direction.
As used herein, the phrases xe2x80x9cheat-shrinkable,xe2x80x9d xe2x80x9cheat-shrink,xe2x80x9d and the like, refer to the tendency of the film to shrink upon the application of heat, i.e., to contract upon being heated, such that the size of the film decreases while the film is in an unrestrained state. As used herein said term refer to films with a total free shrink (i.e., free shrink in the machine direction plus free shrink in the transverse direction), as measured by ASTM D 2732, of at least 30 percent at 120xc2x0 C., more preferably at least 40 percent, still more preferably, at least 50 percent, and, yet still more preferably, at least 60 percent.
As used herein, the term xe2x80x9cmonomerxe2x80x9d refers to a relatively simple compound, usually containing carbon and of low molecular weight, which can react to form a polymer by combining with itself or with other similar molecules or compounds.
As used herein, the term xe2x80x9cco-monomerxe2x80x9d refers to a monomer that is co-polymerized with at least one different monomer in a co-polymerization reaction, the result of which is a copolymer.
As used herein, the term xe2x80x9cpolymerxe2x80x9d refers to the product of a polymerization reaction, and is inclusive of homo-polymers, and co-polymers.
As used herein, the term xe2x80x9chomo-polymerxe2x80x9d is used with reference to a polymer resulting from the polymerization of a single monomer, i.e., a polymer consisting essentially of a single type of mer, i.e., repeating unit.
As used herein, the term xe2x80x9cco-polymerxe2x80x9d refers to polymers formed by the polymerization reaction of at least two different monomers. For example, the term xe2x80x9cco-polymerxe2x80x9d includes the co-polymerization reaction product of ethylene and an xcex1-olefin, such as 1-hexene. However, the term xe2x80x9cco-polymerxe2x80x9d is also inclusive of, for example, the co-polymerization of a mixture of ethylene, propylene, 1-hexene, and 1-octene. The term xe2x80x9cco-polymerxe2x80x9d is also inclusive of random co-polymers, block co-polymers, and graft co-polymers.
As used herein, terminology employing a xe2x80x9c-xe2x80x9d with respect to the chemical identity of a copolymer (e.g., xe2x80x9can ethylene-xcex1-olefin copolymerxe2x80x9d), identifies the co-monomers which are co-polymerized to produce the copolymer.
As used herein, the phrase xe2x80x9cheterogeneous polymerxe2x80x9d refers to polymerization reaction products of relatively wide variation in molecular weight and relatively wide variation in composition distribution, i.e., typical polymers prepared, for example, using conventional Ziegler-Natta catalysts. Heterogeneous polymers are useful in various layers of the film used in the present invention. Although there are a few exceptions (such as TAFMER(trademark) linear homogeneous ethylene-xcex1-olefin copolymers produced by Mitsui, using Ziegler-Natta catalysts), heterogeneous polymers typically contain a relatively wide variety of chain lengths and co-monomer percentages.
As used herein, the phrase xe2x80x9chomogeneous polymerxe2x80x9d refers to polymerization reaction products of relatively narrow molecular weight distribution and relatively narrow composition distribution. Homogeneous polymers are structurally different from heterogeneous polymers, in that homogeneous polymers exhibit a relatively even sequencing of co-monomers within a chain, a mirroring of sequence distribution in all chains, and a similarity of length of all chains, i.e., a narrower molecular weight distribution. Furthermore, homogeneous polymers are typically prepared using metallocene, or other single-site type catalysts, rather than using Ziegler Natta catalysts.
More particularly, homogeneous ethylene-xcex1-olefin copolymers may be characterized by one or more methods known to those of skill in the art, such as molecular weight distribution (Mw/Mn) composition distribution breadth index (CDBI), and narrow melting point range and single melt point behavior. The molecular weight distribution (Mw/Mn), also known as polydispersity, may be determined by gel permeation chromatography. The homogeneous ethylene-xcex1-olefin copolymers useful in this invention generally have (Mw/Mn) of less than 2.7; preferably from about 1.9 to about 2.5; more preferably, from about 1.9 to about 2.3. The composition distribution breadth index (CDBI) of such homogeneous ethylene-xcex1-olefin copolymers will generally be greater than about 70 percent. The CDBI is defined as the weight percent of the copolymer molecules having a co-monomer content within 50 percent (i.e., plus or minus 50%) of the median total molar co-monomer content. The CDBI of linear polyethylene, which does not contain a co-monomer, is defined to be 100%. The Composition Distribution Breadth Index (CDBI) is determined via the technique of Temperature Rising Elution Fractionation (TREF). CDBI determination clearly distinguishes the homogeneous copolymers used in the present invention (narrow composition distribution as assessed by CDBI values generally above 70%) from VLDPEs available commercially which generally have a broad composition distribution as assessed by CDBI values generally less than 55%. The CDBI of a copolymer is readily calculated from data obtained from techniques known in the art, such as, for example, temperature rising elution fractionation as described, for example, in Wild et. al., J. Poly. Sci. Poly. Phys. Ed., Vol. 20, p.441 (1982). Preferably, the homogeneous ethylene-xcex1-olefin co-polymers have a CDBI greater than about 70%, i.e., a CDBI of from about 70% to about 99%. In general, the homogeneous ethylene-xcex1-olefin co-polymers in the multi-layer films of the present invention also exhibit a relatively narrow melting point range, in comparison with xe2x80x9cheterogeneous copolymersxe2x80x9d, i.e., polymers having a CDBI of less than 55%. Preferably, the homogeneous ethylene-xcex1-olefin copolymers exhibit an essentially singular melting point characteristic, with a peak melting point (Tm), as determined by Differential Scanning Calorimetry (DSC), of from about 60xc2x0 C. to about 110xc2x0 C. Preferably the homogeneous copolymer has a DSC peak Tm of from about 80xc2x0 C. to about 105xc2x0 C. As used herein, the phrase xe2x80x9cessentially single melting pointxe2x80x9d means that at least about 80%, by weight, of the material corresponds to a single Tm peak at a temperature within the range of from about 60xc2x0 C. to about 110xc2x0 C., and essentially no substantial fraction of the material has a peak melting point in excess of about 115xc2x0 C., as determined by DSC analysis. Melting information reported are second melting data, i.e., the sample is heated at a programmed rate of 10xc2x0 C./min. to a temperature below its critical range. The sample is then reheated (2nd melting) at a programmed rate of 10xc2x0 C./min. The presence of higher melting peaks is detrimental to film properties such as haze, and compromises the chances for meaningful reduction in the seal initiation temperature of the final film.
A homogeneous ethylene-xcex1-olefin copolymer can, in general, be prepared by the co-polymerization of ethylene and any one or more xcex1-olefins. Preferably, the xcex1-olefin is a C4-C2 xcex1-mono-olefin, still more preferably, a C4-C8 xcex1-mono-olefin. Still more preferably, the xcex1-olefin comprises at least one member selected from the group consisting of butene-1, hexene-1, and octene-1, i.e., 1-butene, 1-hexene, and 1-octene, respectively. Most preferably, the xcex1-olefin comprises octene-1, and/or a blend of hexene-1 and butene-1.
Processes for preparing and using homogeneous polymers are disclosed in U.S. Pat. No. 5,266,075, U.S. Pat. No. 5,241,031, and PCT International Application WO 93/03093, each of which is hereby incorporated by reference thereto, in its entirety. Further details regarding the production and use of homogeneous ethylene-xcex1-olefin copolymers are disclosed in WO-A-90/03414, and WO-A-93/03093.
Still another genus of homogeneous ethylene-xcex1-olefin copolymers is disclosed in U.S. Pat. No. 5,272,236, to Lai, et. al., and U.S. Pat. No. 5,278,272, to Lai, et. al.
As used herein, the term xe2x80x9cpolyolefinxe2x80x9d refers to any polymerized olefin, which can be linear, branched, cyclic, aliphatic, aromatic, substituted, or unsubstituted. More specifically, included in the term polyolefin are homo-polymers of olefin, co-polymers of olefin, co-polymers of an olefin and an non-olefinic co-monomer co-polymerizable with the olefin, such as vinyl monomers, modified polymers thereof, and the like. Specific examples include polyethylene homo-polymer, polypropylene homo-polymer, polybutene homo-polymer, ethylene-xcex1-olefin co-polymer, propylene-xcex1-olefin co-polymer, butene-xcex1-olefin co-polymer, ethylene-unsaturated ester co-polymer, ethylene-unsaturated acid co-polymer, (e.g. ethyl acrylate co-polymer, ethylene-butyl acrylate co-polymer, ethylene-methyl acrylate co-polymer, ethylene-acrylic acid co-polymer, and ethylene-methacrylic acid co-polymer), ionomer resin, polymethylpentene, etc.
As used herein the term xe2x80x9cmodified polyolefinxe2x80x9d is inclusive of modified polymer prepared by co-polymerizing the homo-polymer of the olefin or co-polymer thereof with an unsaturated carboxylic acid, e.g., maleic acid, fumaric acid or the like, or a derivative thereof such as the anhydride, ester or metal salt or the like. It is also inclusive of modified polymers obtained by incorporating into the olefin homo-polymer or co-polymer, by blending or preferably by grafting, an unsaturated carboxylic acid, e.g., maleic acid, fumaric acid or the like, or a derivative thereof such as the anhydride, ester or metal salt or the like.
As used herein, the phrase xe2x80x9cethylene-xcex1-olefin copolymerxe2x80x9d refer to such heterogeneous materials as linear low density polyethylene (LLDPE), linear medium density polyethylene (LMDPE) and very low and ultra low density polyethylene (VLDPE and ULDPE); and homogeneous polymers such as metallocene-catalyzed EXACT(trademark) linear homogeneous ethylene-xcex1-olefin copolymer resins obtainable from Exxon, single-site AFFFNITY(trademark) linear homogeneous ethylene-xcex1-olefin copolymer resins obtainable from Dow, and TAFMER(trademark) linear homogeneous ethylene-xcex1-olefin copolymer resins obtainable from Mitsui. All these materials generally include co-polymers of ethylene with one or more co-monomers selected from C4 to C10 xcex1-olefin such as butene-1, hexene-1, octene-1, etc. in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. The heterogeneous ethylene-xcex1-olefin co-polymer commonly known as LLDPE has a density usually in the range of from about 0.915 g/cm3 to about 0.930 g/cm3, that commonly known as LMDPE has a density usually in the range of from about 0.930 g/cm3 to about 0.945 g/cm3, while those commonly identified as VLDPE or ULDPE have a density lower than about 0.915 g/cm3.
As used herein, the term xe2x80x9cadheredxe2x80x9d is inclusive of films which are directly adhered to one another using a heat-seal or other means, as well as films which are adhered to one another using an adhesive which is between the two films. As used herein, the phrase xe2x80x9cdirectly adheredxe2x80x9d, as applied to layers, is defined as adhesion of the subject layer to the object layer, without a tie layer, adhesive, or other layer therebetween. In contrast, as used herein, the word xe2x80x9cbetweenxe2x80x9d, as applied to a layer expressed as being between two other specified layers, includes both direct adherence of the subject layer between to the two other layers it is between, as well as a lack of direct adherence to either or both of the two other layers the subject layer is between, i.e., one or more additional layers can be imposed between the subject layer and one or more of the layers the subject layer is between.
As used herein, the phrase xe2x80x9cfree shrinkxe2x80x9d refers to the percent dimensional change in a 10 cmxc3x9710 cm specimen of film, when subjected to selected heat (i.e., at a certain temperature), with the quantitative determination being carried out according to ASTM D 2732, as set forth in the 1990 Annual Book of ASTM Standards, Vol. 08.02, pp.368-371. xe2x80x9cTotal free shrinkxe2x80x9d is determined by summing the percent free shrink in the machine direction with the percentage of free shrink in the transverse direction.
As used herein, xe2x80x9cEVOHxe2x80x9d refers to ethylene/vinyl alcohol copolymer. EVOH includes saponified or hydrolyzed ethylene/vinyl acetate copolymers, and refers to a vinyl alcohol copolymer having an ethylene comonomer, and prepared by, for example, hydrolysis of vinyl acetate copolymers, or by chemical reactions with polyvinyl alcohol. The degree of hydrolysis is preferably at least 50%. and more preferably, at least 85%. Preferably, the EVOH comprises from about 28 to about 48 mole % ethylene, more preferably, from about 32 to about 44 mole % ethylene, and even more preferably, from about 38 to about 44 mole % ethylene.
As used herein, the term xe2x80x9cpolyamidexe2x80x9d refers to both polyamide homo-polymers and polyamide co-polymers, also called co-polyamides.
As used herein the term xe2x80x9cco-polyamidexe2x80x9d on the other hand identifies the polyamide an product built from at least two different starting materials, i.e. lactams, aminocarboxylic acids, equimolar amounts of diamines and dicarboxylic acids, in any proportion; this term therefore also encompasses ter-polyamides and, in general, multi-polyamides.